The present invention relates to a method for producing a propylene polymer which includes the use of an alkoxysilane compound having a trialkylsilyl group in the molecular structure thereof, specifically to a method for easily producing an isotactic propylene polymer having dramatically improved melt flowability by characteristically using an alkoxysilane compound having a trialkylsilyl group in the molecular structure thereof as an external electron donor so as to improve the reactivity of hydrogen that is served as a molecular weight regulator.
The catalyst system used in gas phase polymerization, slurry polymerization and bulk polymerization of propylene is generally comprised of a Ziegler type catalyst, alkyl aluminum and an external electron donor. In the general catalyst system for propylene polymerization, it is well known that the external electron donor has a function of improving the isotactic index, i.e. the stereoregularity of the resulted polymer, by selectively poisoning or converting the non-stereoregular active sites present on the surface of a solid catalyst. It means that, depending on the molecular structure of an external electron donor used, the stereoregularity, the activity and the molecular weight distribution of the resulted polypropylene polymer become varied. In this regard, in order to obtain a polypropylene polymer having improved properties as described above, various conventional techniques particularly using various silane compounds as an external donor have been known in this field.
For example, Japanese laid-open patent publication Heisei 3-33105 and Heisei 3-33106 disclose a method for producing polypropylene which has high stereoregularity by using a solid catalyst component synthesized from a supporter which has been obtained from the reaction between silica and dialkyl magnesium, triethyl aluminum and, as an external electron donor, dialkyl dimethoxysilane compound(s). However, when using a dialkyl dimethoxysilane compound as an external electron donor, as in the above-said invention, a problem would occur that the hydrogen reactivity becomes significantly reduced.
As another example, U.S. Pat. No. 4,952,649 discloses a method for producing high stereoregular polypropylene having the isotacticity index of 96˜98%, by preparing a solid catalyst particles obtained from the reaction of a solution of magnesium chloride in 2-ethylhexyl alcohol with titanium tetrachloride and dialkylphthalate at −20˜130° C., and then carrying out bulk polymerization of propylene by using the solid catalyst obtained as above together with a cocatalyst triethyl aluminum and various dialkyl dialkoxysilanes as an external electron donor. Further, in U.S. Pat. No. 6,384,160, a method for producing polypropylene having the isotacticity index of 92˜98% is disclosed by using a solid catalyst component, triethyl aluminum as a cocatalyst and dialkyl dimethoxysilane as an external electron donor, wherein the solid catalyst component is obtained by reacting diethoxymagnesium, titanium tetrachloride and diisobutyl phthalate or diethylphthalate as an internal electron donor.
However, when using the main catalyst component and external electron donor as disclosed in the above-mentioned U.S. patents, the reactivity of hydrogen which is used as a molecular weight regulator would not be sufficient enough to practically prepare polypropylene having the melt flow rate of 50 g/10 minutes or more. Particularly, in practical commercial processes, when introducing a large amount of hydrogen into a polymerization reactor so as to complement the insufficient hydrogen reactivity, there will be a risk of explosion or the like owing to the limitation in designed pressure resistance. It means that the amount of hydrogen being applied into a commercial process is substantially limited. Therefore, in practically operated commercial processes, when using the main catalyst component and the external electron donor disclosed in the above-mentioned US patents, there will be a problem that hydrogen cannot be introduced in an amount sufficient to provide the enough pressure for preparing polypropylene having high melt flowability.
In Japanese laid-open patent publication Sho 57-63311, a method for olefin polymerization comprising the use of phenyl triethoxysilane as an external electron donor is disclosed, but it poses a problem of toxicity owing to phenyl group left from the external electron donor after completing the polymerization process. Further, a method for olefin polymerization using dialkyl dimethoxysilane disclosed in Japanese laid-open patent publication Heisei 2-170803, has a disadvantage that it is impossible to prepare a polymer having high melt index. According to another method for olefin polymerization disclosed in Japanese patent publication Heisei 8-157520, which uses alkyl alkoxysilane substituted with a cycloalkyl group as an internal electron donor, it has a problem of significantly low catalyst activity while achieving relatively higher degree of melt flowability of the resulted polymer.